Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mass of a planet

  1. Sep 24, 2011 #1
    Hi all :)

    Hope the day has been good to you....i am stuck on a question for my uni assignment, and have been stuck on this one for about 3 days to no avail....i don't even know where to start. I'm not too sure whether i need a certain equation for the question or if i have to come up with one using the information in the question...
    If anyone could please help me through it would be greatly appreciated as i've only got 4 days left.....if not thanks for looking anyways :)

    There once was a planet - quite far
    In orbit around a star
    The star’s mass was ms
    The planet’s mass you assess
    Using the orbital radii, r

    You can assume the radius of the planet’s and the star’s circular orbits around the
    centre of mass of the system are rp = 1.5 × 108 km and rs = 1.0 × 106 km, respectively, with
    ms = 2.0 × 1030 kg.

    Calculate the mass of the planet, mp, showing all your workings and giving your answer in scientific notation to the appropriate number of significant figures.

    Any help would be amazing as i'm at my wits end !
    Many Thanks,

  2. jcsd
  3. Sep 24, 2011 #2
    So far all the info i have is that;

    a) the star is 1 million miles from the centre of mass
    b) the planet is 150 million miles from the centre of mass; and
    c) both bodies are in circular orbits to the centre.....

    so i'm not too sure how to use this data to calculate the answer....

  4. Sep 24, 2011 #3


    User Avatar
    Homework Helper

    I think I would treat it as a simple centre of Mass problem. With the planet 150 times forther from the centre of mass I think it should simple be 1/150 th of the mass of the Sun.
  5. Sep 24, 2011 #4
    ok thanks very much PeterO :)

    i have done a calculation but am not too sure whether this is correct at all......

    mp = rp x rs


    mp = 1.5 x 108km x 1.0 x 106km
    2.0 x 1030kg

    mp = 150 000 000 000 000
    2.0 x 1030kg

    mp = 0.000 000 000 000 000 075 ??

    ok, so i am also now thinking that this is incorrect....gah !

    thanks for the help though, much appreciated :)
  6. Sep 24, 2011 #5


    User Avatar

    Staff: Mentor

    How did you end up with miles as units when the problem appears to use kilometers?
  7. Sep 24, 2011 #6
    oops !

    my bad, sorry bout that i did mean kilometres....sorry been at this for a while now and my brain is kinda mushy....thanks for pointing that out
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook